Flow Control in Biomedical Microdevices using Thermally Responsive Fluids 119
different parameters.
27,5357
Current applications for biocompatible smart gels
include artificial muscles,
36,58,6062
and drug delivery.
37,6367,69
In a fundamentally different approach, thermally responsive polymer solu-
tions have also been used as the working fluid in microfluidic devices. These
polymer solutions solidify reversibly upon heating, so that selective heating of
specific areas of a microfluidic system then causes localized gel formation to
temporarily block a channel. Shirasaki and co-workers
72
used external infrared
lasers for heating and localized gel formation and channel blockage for cell sorting;
however, this approach depends significantly on transparency and absorbance
of the materials involved. In a slightly different approach, electric heaters were
integrated into a microflow system to achieve rapid and controlled heating of the
fluid and subsequent
53
valving, taking advantage of the fast time scales for thermal
diffusion in microdevices as discussed in Section 6.2.
6.4 THERMALLY RESPONSIVE FLUIDS FOR MICROFLOW CONTROL
6.4.1 Temperature Responsive Materials
Temperature responsive fluids (TRF) or temperature responsive hydrogels are
among the most commonly studied and used materials for environmentally re-
sponsive microsystems.
2635
Temperature responsive hydrogels react to the local
temperature through phase transition or swelling and this thermo sensitivity can
be used as an actuation method. Based on their reversible actuation mechanisms,
smart hydrogels have been widely used in microvalves and microsensors as
mentioned earlier,
27,75
. Recently, these materials have gained increasing attention
because of their potential application to biomedical systems and devices for tissues
engineering, cell handling and drug delivery.
28,29,37,65,66,7687
Thermally responsive fluids that undergo a phase change have a critical
solution temperature (CST) at which the phase transition occurs, which can take
place in form of a sol-gel transition upon heating or cooling. This transition occurs
through interaction of polymer and water molecules in the case of TRFs exhibiting
a critical phase separation temperature; otherwise, phase transition at a critical
gelation temperature can be caused by intermolecular forces between polymer
groups in the presence of water in form of hydrogen bonding and hydrophobic
interactions due to the presence of hydrophobic groups such as methyl, ethyl
and propyl.
29
Among the first group, poly(N-iso-propylacrylamide) (PNIPAAm)
and also poly(N, N-diethylacrylamide) (PDEAAm) are the most extensively used
materials.
88
The fundamental behavior of NIPAAm has been extensively studied
and the possibility to form copolymers of NIPAAm by using other monomers
has been demonstrated.
29,88,89
Gelation based on sol-gel transition of polymer
solutions through hydrophobic/hydrophilic interaction on the other hand can
be achieved with certain types of block copolymers made of poly-ethylene-oxide
(PEO) and poly-propylene-oxide (PPO). Pluronics (or Poloxamers) and Tetronics
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